In modern society, many electronic appliances are used and various data is generated and employed, requiring a memory device to store the data. Various memory devices produced and used today each has different advantages and disadvantages, and is selected depending on the data to be stored and used.
For example, a volatile memory that loses its memory content when the power is turned off includes a DRAM and an SRAM. The volatile memory has limited applications because of the volatility; however, it is used as a main memory device or a cash memory of a computer taking advantage of a short access time. As each memory cell has a small size, a large-capacity DRAM can be produced easily, though it is controlled in a complex manner and consumes much power. Meanwhile, an SRAM includes a memory cell constituted by a CMOS and is easily manufactured and controlled, though a large-capacity SRAM is produced with difficulty since one memory cell needs six transistors.
A nonvolatile memory that holds its memory content even after the power is turned off includes: a rewritable memory where data can be rewritten many times; a write-once memory where data can be written by a user only once; and a mask ROM where data content is determined in the manufacture of the memory and cannot be rewritten thereafter. As the rewritable memory, there are an EPROM, a flash memory, a ferroelectric memory, and the like. The EPROM allows an easy writing of data and unit cost per bit is relatively low, though dedicated program device and eraser for writing and erasing are required. The flash memory and the ferroelectric memory allow rewriting on a substrate used, have a short access time, and consume low power, though manufacturing steps of a floating gate and a ferroelectric layer are required to increase unit cost per bit.
Each memory cell of a write-once memory is constituted by a fuse, an antifuse, a cross pointer diode, an OLED (Organic Light Emitting Diode), a bistable liquid crystal element, and other devices whose state is changed by heat or light. In general, a memory device stores data by selecting one of the two states of each memory cell. The write-once memory device is manufactured with all memory cells having a first state, and only memory cells specified by a writing operation are changed to a second state. The change from the first state to the second state is irreversible and the changed memory cell cannot be restored.
Manufacturing steps of a write-once memory have limited temperature and materials, thus it is not formed on a silicon substrate in many cases. That is, a write-once memory is manufactured in completely different steps than a central processing unit (hereinafter referred to as a CPU), an arithmetic circuit, a rectification circuit, a control circuit and the like (hereinafter collectively referred to as other functional circuits to be distinguished from a write-once memory), which are generally formed on a silicon substrate. For example, an antifuse write-once memory has a wiring, an antifuse layer, and a control element, which are formed on a plastic or metal substrate (see Patent Document 1). The memory device manufactured in this manner achieves low cost, large capacity, low power consumption, and short access time. In the case of forming a semiconductor device having a certain function, however, a memory does not operate by itself and other functional circuits are necessarily required. Therefore, it is necessary to form a memory such as a write-once memory and other functional circuits separately.
In recent years, an IC tag has been known as an example of a semiconductor device where a memory and other functional circuits are integrated on the same silicon substrate. An IC tag includes memories such as an SRAM, a mask ROM, a flash memory, and a ferroelectric memory. A mask ROM is a memory where data content is determined in the manufacture of the memory and cannot be rewritten by an IC tag user. In addition, a piece of data is determined by one photomask; therefore, the mask ROM requires as many photomasks as kinds of data. Thus, the mask ROM is not practical for cost reasons.
A flash memory and a ferroelectric memory require additional steps for forming a floating gate and a ferroelectric layer in a gate insulating film. Meanwhile, all the circuits other than the memory in an IC tag can be obtained by CMOS manufacturing steps.
In recent years, technologies for forming a thin film transistor (hereinafter referred to as a TFT) on an insulating substrate have been actively developed to manufacture a display device such as a liquid crystal display and an EL display. For example, a driver circuit for displaying images and a pixel portion are formed on the same substrate using TFTs. Since an insulating substrate is not capacitively coupled to a wiring, high speed operation of a circuit can be achieved. Accordingly, various functional circuits such as an arithmetic circuit and a memory circuit are proposed to be formed using TFTs. Another advantage of forming functional circuits on an insulating substrate is cost saving. A glass substrate and a plastic substrate are quite inexpensive as compared to a silicon substrate. Further, it is possible to use an insulating substrate with a larger area than a silicon substrate that is limited to a small area. Thus, the number of products manufactured on an insulating substrate increases than that manufactured on a silicon substrate, leading to a very inexpensive semiconductor device.
A memory device formed using TFT manufacturing technologies includes a mask ROM, an SRAM, and a flash memory. An SRAM including TFTs can be easily formed on the same substrate as other functional circuits, though it has limited applications because of the volatility. A mask ROM is not practical since different photomasks are required for different data. A flash memory requires additional steps for forming a floating gate, though other functional circuits such as an arithmetic circuit on an insulating substrate can be formed by TFT manufacturing steps.
As set forth above, the invention is made in view of the two technologies: a technology for forming a memory device and a technology for forming a circuit on an insulating substrate such as a glass substrate or an insulating surface.